The present invention relates to a manganese based catalyst for use in a metal air electrochemical cell, and more particularly, relates to the composition and manufacture of an air electrode comprising said catalyst.
Metal-air electrochemical cells utilize oxygen from the ambient air as a reactant in an electrochemical reaction to provide a relatively lightweight power supply. Generally described, a metal-air cell includes an air-permeable cathode and a metallic anode separated by an aqueous electrolyte. During the operation of a metal-air cell, oxygen from the ambient air is reduced at the cathode to form hydroxide ions. The metal is oxidized at the anode and reacts with the hydroxide ions, such that electrons are released to provide electrical energy.
Most air electrodes are either inherently or by purposive implementation, catalytically active. Although effective air electrodes are available, there remains a need for air electrodes with enhanced catalytic activity. An important factor in optimizing electrode performance is catalyst particle dispersion. The more uniformly the catalyst is dispersed and the smaller the size of the catalyst particles, the higher the activity of the air electrode. With conventional methods of catalyst production it is difficult to reduce the particle size of the catalyst enough to uniformly distribute it throughout the active layer of the electrode.
The present invention fulfills the above-described need by providing a method of producing a manganese based oxygen reduction catalyst for use in an air electrode using micelle encapsulation. This invention also encompasses the resulting catalyst, a cathode comprising said catalyst and methods of making a cathode comprising said catalyst. As a result of the micelle method, the catalyst has particles which are submicron and easily dispersed throughout a catalyst support.
More particularly, the present invention relates to a method of producing a manganese based oxygen reduction catalyst wherein a first solution comprising a first surfactant and manganese nitrate and a second solution comprising a second surfactant and ammonium hydroxide are prepared. The two solutions and activated carbon are admixed and then sonicated to form micelles comprising manganese and carbon. A precipitate is obtained by centrifugation and heated at a temperature between 450xc2x0 C. to 700xc2x0 C. to form a manganese oxide. The micelle size is submicron and the resulting size of manganese adsorbed onto the carbon surface is also submicron.
The present invention also relates to an air cathode comprising the above-described catalyst. More particularly, the air cathode comprises an active layer including the oxygen reduction catalyst of the present invention. In addition, the air cathode includes a current collector in electrical contact with the electrode. The active layer of the air cathode further comprises carbon black and a non-wetting agent/binder such as polytetrafluoroethylene.
The air cathode of the present invention is made according to a process comprising forming an active layer made with the oxygen reduction catalyst of the present invention, and positioning a current collector in electrical contact with the air cathode. More particularly, the active layer is formed by admixing the manganese based catalyst with an adsorptive particulate material, such as carbon black, and a non-wetting agent/binder, such as polytetrafluoroethylene.